.parent {
    width: 400px;
    background-color: orange;
    padding: 8px;
}

.child {
    width: 100%;
    box-sizing: border-box;
    background-color: lightcyan;
    padding: 8px;
}

.grand {
    width: 100%;
    /* box-sizing: border-box; */
    background-color: lightgray;
}